Volkswagen used this effect a couple of years ago at their Detroit Auto Show press event. Several large LCD screens were modified by removing the polarizing layer, and the press in attendance (several hundred) were given standard polarized sunglasses. It was definitely attention getting.

For all the preparation, no one noticed that the reflection on a shiny floor made the images visible (if inverted) to everyone.

Shine light through two polarizing filters oriented at 90° to each other, and no light gets through. But put a third filter between them, at 45° to each of the existing filters, and amazingly enough — some lights gets through!